This invention relates to a light measuring device for use in a single reflex camera, and more particularly to a light measuring device located in contact with the pentagonal prism in the single reflex camera and receiving part of the scene light which has been transmitted through an objective lens, thereby measuring the brightness of the scene light.
Two types of light receiving devices for measuring scene light which has been transmitted through an objective lens are known, i.e., a light receiving device of the type in which part of the light which is directed to an ocular is introduced to a light receiving element by an optical member such as a half-mirror and the device of the type in which such part of the scene light scattered on the focussing plate but not introduced to the objective lens is introduced to the light receiving device.
In addition, it is a known practice to locate the light receiving devices of the aforesaid two types adjacent a pentagonal prism, respectively, to introduce scene light to the light receiving device.
In this respect, it is required that the scene light from the entire scene to be photographed be uniformly introduced to the light receiving device of this type. Thus, if the light from the respective portions of a scene is introduced to the light receiving device non-uniformly, the photoelectric element in the light receiving device fails to produce the proper light measuring outputs corresponding to the brightness of the entire scene. In general, the image of a sky portion of the scene focussed in the front portion of a focussing plate is brighter than the ground image in the rear portion of the focussing plate. Thus, it is preferable that the scene light focussed in rear portion of the focussing plate be introduced in greater quantity into the light receiving device than that of the scene light focussing in the front portion of the focussing plate.
In contrast thereto, it would be advantageous if, as in the former instance, wherein part of the light beams in the optical path of a viewfinder are introduced to the light receiving device placed adjacent the pentagonal prism, uniform scene light throughout the entire scene may be introduced to the light receiving device. However, this poses disadvantages in that the image in a viewfinder to be observed is partly or entirely in the sense of image area darkened. Thus, if the brightness of an image is desired to be increased, then there results a decrease in the quantity of the light to be introduced to the light measuring device, with the accompanying decrease in a light measuring output, thus lowering the accuracy of light measurement.
Turning to the latter light receiving device, this device is to measure the scene light which is not incident on an ocular lens. Thus, this can introduce a great quantity of scene light to the light receiving device without darkening the viewfinder image. However, receiving the light other than that adapted for the view finder, the device is more responsive to the light from the part of image closer to the device than to that distant from the device, and consequently, the device can not measure scene light with a desired sensitivity distribution. For this reason, with the light receiving device of this type for use in measuring light beams, there should be provided two light receiving devices in symmetric fashion with respect to the optical axis of the optical path of a viewfinder or the center of an image, but adjacent the pentagonal prism located out of the optical path of a viewfinder, while respective photoelectric elements of both the light receiving devices should be connected in series or parallel to each other to thereby obtain a light measuring output resulting from uniform measurement of light beams from the entire scene. Meanwhile, it is desired to render the size of a camera compact, for obtaining good portability. In this sense, it is not advantageous to use two light receiving devices. The pentagonal prism as used in a single reflex camera is large in its volume and weight. This leads to the increase in the volume and weight of the viewfinder and brings the position of gravity of the camera high in the normal attitude thereof. This is not desirable from the viewpoints of stability of a camera.